当字段有内容和没有内容时，主题中的Flutter Form标签样式需要具有相同的大小

Form(
  child: Column(
    children: [
      TextFormField(
        decoration: InputDecoration(
          labelText: 'Username',
        ),
      ),
      TextFormField(
        decoration: InputDecoration(
          labelText: 'Password',
        ),
      ),
    ],
  ),
),

Form(
  child: Column(
    children: [
      Container(
        height: 50, // 设置Container的高度
        child: Align(
          alignment: Alignment.centerLeft,
          child: TextFormField(
            decoration: InputDecoration(
              labelText: 'Username',
            ),
          ),
        ),
      ),
      Container(
        height: 50, // 设置Container的高度
        child: Align(
          alignment: Alignment.centerLeft,
          child: TextFormField(
            decoration: InputDecoration(
              labelText: 'Password',
            ),
          ),
        ),
      ),
    ],
  ),
),